In U.S. Pat. No. 3,645,201 there is described and illustrated a multi-cylinder printing machine in which each printing cylinder is detachably mounted on a hub member of the machine, the cylinder having axial and radial abutments in engagement with corresponding axial and radial abutments on the hub member. The axial and radial abutments on each hub are in register with the corresponding axial and radial abutments on the other hub members whereby the printing cylinders may be interchanged or exchanged with other cylinders without loss of registration of the images printed by the cylinders. For convenience, a multi-cylinder flexographic printing machine having such axial and radial abutments on the hub members and cylinders will be referred to hereinafter as a multi-cylinder printing machine of the type described.
In the machine of the type described, it is of course essential that the printing plates on the cylinders be accurately positioned relative to the radial and axial abutments on the cylinders to ensure registration of the images printed by the different cylinders.
The printing plates are generally secured to the printing cylinders by adhesive so that the plates can readily be removed and refitted to the cylinders. Hitherto, the printing plates have been positioned on the cylinders by trial and error, the machine being used to print with the plates and any errors in registration of the images printed by the plates corrected by removing and refitting the plates. This is however a slow and laborious operation since, in addition to aligning the plates accurately relative to the abutments, it is frequently necessary to stretch some of the plates. Flexographic printing plates cannot be made with great precision due to the fact that they are molded from flexible material under heat and the shrinkage which occurs on cooling of the mold is not identical for all plates.